1. Field of the Invention
This invention relates to a powder additive for powder metallurgy, to be mixed in an iron-based powder which is a primary raw material powder to obtain a powder mixture for powder metallurgy, such as alloying powder or machinability improving powder or the like. Also, this invention relates to a method for manufacturing the powder additive for powder metallurgy. Further, this invention relates to an iron-based powder mixture for powder metallurgy wherein the powder additives for powder metallurgy are bonded to the surface of iron powder by an organic binder, and a method of producing thereof.
2. Description of the Related Art
An iron-based powder mixture for powder metallurgy generally is an iron-based powder of iron powder or alloy steel powder or the like, to which powder additives for powder metallurgy and a lubricant are added as needed. Examples of the powder additives for powder metallurgy added include alloying powders such as copper powder, graphite powder, iron phosphide or the like, machinability improving powders such as MnS powder, BN powder, CaF powder or the like. Examples of lubricants include zinc stearate, aluminum stearate, lead stearate and the like.
In recent years, there have been increasing demands for reduction in costs of sintered materials and, thus, reduction in manufacturing costs. For example, preventing segregation of raw material powders such as the iron-based powder, powder additives, and lubricant, reduces dimensional irregularity at the time of compact sintering. Consequently, the costs necessary for correcting the dimensions of the sintered material following sintering by the cutting process can be reduced. Accordingly, various endeavors have been made to prevent segregation of the iron-based powder mixture for powder metallurgy.
Further, there have also been demands for reduction in manufacturing costs of the iron-based powder mixture for powder metallurgy itself.
Using an organic binder to bond powder additives to the iron-based powder is known to be effective in preventing segregation of the iron-based powder mixture for powder metallurgy. The following are well-known techniques:
(1) Wet mixing: Powder additives, the iron-based powder, and the lubricant are mixed with a liquid wherein an organic binder has been dispersed or dissolved, from which the dispersion medium or solvent is dried (e.g., Japanese Patent No. 2,582,231 (Claims), Japanese Examined Patent Application Publication No. 5-27682 (Claims)).
(2) Dry mixing: The powder additives, the iron-based powder, and a solid organic binder are heated while mixing, and the organic binder is melted and then cooled to bind the powder additives for powder metallurgy and the iron-based powder together. A particularly preferred technique is to mix in a solid lubricant, and to heat and melt at least part of the solid lubricant to serve as an organic binder (e.g., Japanese Unexamined Patent Application Publication No. 2-57602 (Claims), Japanese Unexamined Patent Application Publication No. 3-162502 (Claims)).
FIG. 2 is a model diagram of the iron-based powder mixture for powder metallurgy obtained by the above-described wet mixing method and dry mixing method. Normally, powder additive 7 is formed of a powder additive particle proper 1, which binds to the surface of iron-based powder 3 by the additionally-mixed organic binder 2.
However, with either method, increasing the amount of organic binder added to sufficiently prevent segregation inevitably leads to an increase in useless binder 4 which does not contribute to binding of the iron-based powder and the powder additives, but simply adheres to the surface of the powder additive or the iron-based powder, causing problems such as a decrease in green density because useless binder occupies volume that inhibits the iron base powder. Also, there is an increase in useless binder floating free, which does not adhere to the raw material powders. Accordingly, the above methods do not sufficiently improve segregation of iron-based powder mixture for powder metallurgy.